The present invention relates to method and apparatus for automatic monitoring and control of the chemical etching of quartz blanks. In this process, the resonance frequency of the blanks is adjusted by immersing them in an etching solution. As the etchant thins the blanks, their resonance frequency increases.
The etch process includes two different stages:
A) an initial "nonlinear" stage, during which the blank frequency increases at a fast, nonlinear rate versus time. In this stage, the blank surface is cleaned of loose particles left from the preceding process step of lapping. The rate of frequency change is proportional to the initial roughness of the blank surface. PA1 B) a subsequent "linear" stage, during which the blank frequency increases at a slower, linear rate versus time. In this stage, the blank thickness is reduced at a constant rate, provided the etchant's etch rate is constant. PA1 1) monitoring the etch rate of an etchant. In this application, the monitor blank frequency is monitored continuously in order to determine the etchant's etch rate. From this and the etch load targets specifications for the etch load one can determine the time required for controlling the etching of etch loads in the "linear" etch stage. The monitor blank is typically designed for long etch exposure, so that it can be used over multiple etch runs. PA1 2) monitoring the etching of an etch load. In this application, the monitor blank frequency is monitored in order to determine its change during an etch run.
In present practice, the etchant's etch rate cannot be held exactly constant because it is sensitive to variables such as temperature and concentration of the etchant. Therefore, present etching is a trial-and-error process: Typically, the blanks are exposed to the etchant for a predetermined time, after which they are cleaned, measured in air and, if they don't meet the target frequency, re-etched. This process is labor intensive and not amenable to precise frequency adjustment.